'atween' originates from Middle English, specifically the word 'atweene' (or 'atwēne'), where the prefix 'a-' functioned like 'on'/'in' and 'tween' is related to 'two' (the sense of separation between two items).

'atween' developed as a dialectal/colloquial form in Middle English from older forms related to Old English 'betweonan'/'betwēonum'; over time the standard modern English form 'between' became dominant and 'atween' fell out of standard usage.

Initially and historically it meant 'in the middle of; between' and this core meaning has remained, though the form 'atween' became archaic and is now largely dialectal or literary.